1. Field of the Invention
This invention relates to a thin-film superconductor. This invention also relates to a method of fabricating a thin-film superconductor.
2. Description of the Prior Art
J. G. Bednorz and K. A. Muller discovered a high-Tc superconductor including a perovskite compound of the Ba--La--Cu--O system (J. G. Bednorz and K. A. Muller: Zeitshrift Fur Physik B, Condensed Matter, Vol. 64, 189-193, 1986).
H. Maeda, Y. Tanaka, M. Fukutomi, and T. Asano discovered a superconductor of the Bi--Sr--Ca--Cu--O system which had a temperature Tc of higher than 100K (H. Maeda, Y. Tanaka, M. Fukutomi, and T. Asano: Japanese Journal of Applied Physics, Vol. 27, L209-L210, 1988).
There are several different crystal phases of Bi--Sr--Ca--Cu--O superconductors which have different temperatures Tc respectively. Generally, it has been difficult to make a thin film of a Bi--Sr--Ca--Cu--O superconductor in a crystal phase having a temperature Tc of higher than 100K. The reason for the difficulty is as follows. Bi--Sr--Ca--Cu--O compounds have laminated structures in which quasi-perovskite layers of Sr--Ca--Cu--O extend between layers of Bi.sub.2 O.sub.2. The quasi-perovskite layers usually assume several different phases so that the compounds tend to be out of single crystals. Thus, it is hard to make a thin-film single crystal of a Bi--Sr--Ca--Cu--O compound having a temperature Tc of higher than 100K.
U.S. Pat. No. 5,135,906 discloses a superconducting thin film of the Bi--Sr--Ca--Cu--O system, and a Bi.sub.2 O.sub.3 buffer layer interposed between the superconducting thin film and a substrate of MgO, SrTiO.sub.3, or YSZ.
Japanese published unexamined patent application 2-172821 discloses a thin-film superconductor of the Bi--Sr--Ca--Cu--O system which extends on a composite oxide substrate of (Bi.sub.2 O.sub.2).sup.2+ (A.sub.n-1 R.sub.n O.sub.3n+1).sup.2- where "A" denotes perovskite-structure A-site ion selected from K, Na, Li, Bi, Mg, Sr, Ca, Ba, Y, La, and other rare earth metal elements, and "R" denotes perovskite-structure B-site ion selected from Ti, Nb, Cu, Ni, and Cr. An example of the substrate is made of Bi.sub.4 Ti.sub.3 O.sub.12. In an embodiment disclosed by Japanese application 2-172821, the thin-film superconductor is epitaxially formed on a (110)-plane of the substrate. In another embodiment disclosed by Japanese application 2-172821, the substrate is cut along a plane inclined with respect to the c-axis, and the thin-film superconductor is formed on that plane of the substrate. In Japanese application 2-172821, the superconductor and the substrate are similar in crystal structure, so that the superconductor can be well orientated according to the orientation of the substrate and hence the superconductor can easily be in a desired single phase.